(1) Field of the Invention
This invention relates to a method for activating an alkaline storage cell employing a non-sintered type nickel positive electrode comprising conductive powder as an activator for active material.
(2) Description of the Related Arts
As disclosed in Japanese Laid-open Patent Application No. 53-51449, conductive powder such as a cobalt compound has been added to active material of the non-sintered type nickel positive electrode in an alkaline storage cell as a conventional method for enhancing the utility of the active material. Among various kinds of cobalt, metallic cobalt is well known to be a most effective additive to that purpose. This is because metallic cobalt, which is conductive itself, is converted into more conductive cobalt oxyhydroxide during charging, thereby further enhancing the conductivity among active material particles.
However, mere addition of metallic cobalt powder does not contribute to fully enhance the utility of the active material. The powder must be oxidized to a preferable extent to achieve this purpose.
Meanwhile, Japanese Laid-open Patent Application No. 64-21864 discloses a method of fully drawing out the ability of metallic cobalt to enhance the utility of active material. According to the disclosed method, a cell employing a nickel positive electrode containing metallic cobalt and a yet-formed negative electrode is charged at a low current in the oxidation area of metallic cobalt followed by the charge at a normal current. However, the inventors of the present invention tried this method and found the oxidation of metallic cobalt powder could not proceed favorably for enhancing the utility of the active material.
To solve this problem, the inventors completed the following batteries, the applications on which were filed to Japanese P.T.O:
(1) An alkaline storage battery which employs a non-sintered type nickel positive electrode comprising a substrate and a mixture. The mixture contains nickel hydroxide powder having a surface layer of nickel oxyhydroxide and a powder of a bivalent cobalt compound (Japanese Laid-open Patent Application No. 4-94058).
(2) An alkaline storage battery which employs a non-sintered type positive electrode comprising a substrate and active material. A cobalt compound whose cobalt has a valence of two or less and a higher oxidized cobalt compound whose cobalt has a valence of three or more are added to the active material (Japanese Laid-open Patent Application No. 4-109557).
In the application (1), nickel oxyhydroxide, which can work as active material, is used as an oxidant to chemically encourage the oxidation of the metallic cobalt powder.
In the application (2), the dischargeable higher oxidized cobalt compound powder whose cobalt has a valence of three or more is used as an oxidant to chemically encourage the oxidation of the metallic cobalt powder, thereby enhancing the utility of the active material.
However, an alkaline storage battery constructed in accordance with the application (1) or (2) does not always bring out the ability of the nickel oxyhydroxide powder or the cobalt compound powder added to the positive electrode to encourage the oxidation of the metallic cobalt powder, and as a result, the ability of the metallic cobalt powder to enhance the utility of an active material may not be fully drawn out. Hence, it is required to find out conditions under which these oxidants can preferably progress the oxidation of the metallic cobalt.